Carbonic anhydrases (CA, EC 4.2.1.1, also termed carbonate dehydratases) catalyze the inter-conversion between carbon dioxide and bicarbonate [CO2+H2O⇄HCO3−+H+]. The enzyme was discovered in bovine blood in 1933 (Meldrum and Roughton, 1933, J. Physiol. 80: 113-142) and has since been found widely distributed in nature in all domains of life. These enzymes are categorized in three distinct classes called the alpha-, beta- and gamma-class, and potentially a fourth class, the delta-class. These classes evolved from independent origins (Bacteria, Archaea, Eukarya) and have no significant sequence or structural identity, except for single zinc atom at the catalytic site (for review see Tripp et al., 2001, J. Biol. Chem. 276: 48615-48618). For alpha-CAs more than 11 isozymes have been identified in mammals. Alpha-carbonic anhydrases are abundant in all mammalian tissues where they facilitate the removal of CO2. Beta-CAs are ubiquitous in algae and plants where they provide for CO2 uptake and fixation for photosynthesis. The gamma-class of CAs is believed to have evolved first. The only gamma-CA that has been isolated and characterized so far is from the Archaeon Methanosarcina thermophila strain TM-1 (Alber and Ferry, 1994, Proc. Natl. Acad. Sci. USA 91: 6909-6913), however many gamma-type carbonic anhydrases have been proposed by Parisi et al., 2004, Plant Mol. Biol. 55: 193-207. In prokaryotes genes encoding all three CA classes have been identified, with the beta- and gamma-class predominating. Many prokaryotes contain carbonic anhydrase genes from more than one class or several genes of the same class (for review see Smith and Ferry, 2000, FEMS Microbiol. Rev. 24: 335-366; Tripp et al., 2001. J. Biol. Chem. 276: 48615-48618).
Carbon dioxide (CO2) emissions are a major contributor to the phenomenon of global warming. CO2 is a by-product of combustion and it creates operational, economic, and environmental problems. CO2 emissions may be controlled by capturing CO2 gas before emitted into the atmosphere. There are several chemical approaches to control the CO2 emissions. However, many of these approaches have draw backs such as high energy consumption, slow processes, and use of ecological questionable or toxic compounds.
An enzyme based solution using the capability of carbonic anhydrase to catalyse the conversion of CO2 to bicarbonate at a very high rate (turnover is up to 105 molecules of CO2 per second), takes care of the speed and environmental issues in relation to CO2 capture. Technical solutions for extracting CO2 from gases, such as combustion gases or respiration gases, using carbonic anhydrases have been described in WO 2006/089423, U.S. Pat. No. 6,524,842, WO 2004/007058, WO 2004/028667, US 2004/0029257, U.S. Pat. No. 7,132,090, WO 2005/114417, U.S. Pat. No. 6,143,556, WO 2004/104160, US 2005/214936. Generally, these techniques operate by bringing a soluble or immobilized carbonic anhydrase into contact with CO2 which either may be in a gas phase or a liquid phase. The carbonic anhydrase catalyses the conversion of CO2 into bicarbonate and/or carbonate ions. The ions may either be utilized to facilitate growth of algae or other microorganisms, to induce a pH change in a surrounding medium or supply buffering capacity, to provide bicarbonate/carbonate as an active agent for subsequent chemical processes, or precipitated as a carbonate salt, or converted back into pure CO2, which can then be used (for example in enhanced oil recovery, for production of urea, for food and beverage processing, or to supply CO2 to greenhouses), released (for example from a contained life support environment such as a submarine or spacecraft), compressed (for example for transportation through pipelines), or stored under compression (such as in geological or deep oceanic formations).
Mammalian, plant and prokaryotic carbonic anhydrases (alpha- and beta-class CAs) generally function at physiological temperatures (37° C.) or lower temperatures. The temperature of combustion gasses or the liquids into which they are dissolved may, however, easily exceed the temperature optimum for the carbonic anhydrase used to capture the CO2. Thus, one of the drawbacks of using enzyme based solutions is that extensive cooling may be need prior to contacting the CO2-containing gas/liquid with the carbonic anhydrase, and cooling is an energy consuming process.